The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to correct gradient field distortion where an object moves with respect to the gradient non-linearities.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In MR imaging, magnetic field gradients are used to spatially encode objects. A magnetic field gradient is a linear variation along any of the principal directions of the Bz field. Non-linearities of the magnetic field gradient cause geometric distortion or xe2x80x9cwarpingxe2x80x9d of the resulting image.
It is highly desirable to extend the available imaging field-of-view (FOV) images by continuous or stepped table motion. These techniques attempt to image in the region where the gradients are mostly linear to minimize errors caused by gradient non-linearities. Such errors result in ghosting and/or blurring of the resulting images. The principal goal of acquiring images while the table is moving is to extend the usable imaging FOV beyond that which is normally achievable. However, to date the issue of correcting for gradient distortion in the presence of continuous or stepped table motion has not been adequately resolved. Previous approaches have focused on imaging over a relatively narrow region of the gradient coil where the gradients are substantially linear, thereby reducing the need for correction. However, by increasing the imaging volume to include regions of gradient non-linearity, the acquisition time for these types of scans can be greatly reduced.
In moving table imaging, the subject passes through different physical locations in the magnet during acquisition. Therefore, the subject experiences different amounts of gradient non-linearity as the subject moves from iso-center to the periphery of the gradient field. Thus, the subject is encoded with different positional errors during movement through the magnetic field. These errors can cause blurring and ghosting in the resulting images in addition to geometric distortion. That is, if the table is moving continuously during data acquisition, then each point in k-space is acquired at a different location in the sample being image. This means that each point in the subject experiences different gradient fields over the course of the data acquisition and a correspondingly different amount of distortion.
For the special case of frequency encoding along the direction of motion each phase-encoding step is acquired at a different table position corresponding to a different location in the object being imaged. In this technique, the data is first Fourier transformed along the frequency-encoding direction resulting in hybrid data. Each phase-encoding in this hybrid data can then be registered by calculating the pixel offset from the pulse sequence repetition time (TR) and the table velocity (v) and applying the appropriate shift. Further Fourier transform(s), the number of which is based on whether a 2D or 3D image is being reconstructed, can then be performed on the entire hybrid data set after the appropriate shifts have been applied to each of the phase-encodings. While this technique has proven to provide adequate images in many applications, it could be improved by opening up the FOV to include regions of increased gradient non-linearity and/or could benefit from higher quality images if a gradient non-linearity correction were employed.
It would therefore be desirable to have a method and apparatus to compensate for gradient non-linearity where the gradients vary. A specific implementation of which is moving table imaging.
The present invention relates to a system and method of compensating for gradient field non-linearities to allow large FOV MR imaging using continuous or stepped table motion.
Ideally, in conventional imaging, data is acquired in the presence of linear gradients. However, any deviation from this ideal linearity can cause errors in the final image. That is, it is assumed that the field strength is directly proportional to the distance from the magnet iso-center along the gradient direction. Any variation from this linearity introduces an error such that the resulting image is distorted. Given knowledge of the error in the gradient field, or the deviation from linearity, an approximation of the ideal image can be calculated. The present invention includes a method and apparatus to correct for gradient field distortions. The invention is particularly applicable in moving table imaging where a single extended image or a series of smaller images comprising a larger FOV is desirable. The invention includes acquiring MR data in motion in the presence of gradient non-linearities, transforming the MR data acquired into the image domain, and then applying a warping correction function to the transformed MR data. The warp-corrected MR data is then corrected for motion induced during the MR acquisition. The data may be processed point-by-point, line-by-line, or some other sub-portion of the entire MR data acquired, and processed to minimize the amount of motion correction needed. Based on table velocity or acquisition sequence applied, the data is partitioned based on a common motion correction factor, and after correcting for motion, the data is accumulated to build up a final image.
In the present technique, each data set is corrected separately for gradient distortion by first converting it to an image. To place the data in the image domain, the data is first Fourier transformed, preferably along the frequency encoding direction. In moving table MRI, the frequency encoding direction is preferably along the direction of table motion. Additionally, a second Fourier transform, or a second and third in the case of 3D imaging, is also done for each phase-encoding point or line. This can be done either by applying a Fourier transform to a matrix having the current phase-encoding data therein and the remainder filled with zeros or using the basis that phase-encoding step is a delta function along the phase-encoding direction that corresponds to a unique phase modulation. The resulting image can then be corrected for gradient distortion using predetermined gradient error maps. After correction, the data is shifted by a motion offset and added to the previous data. This process is repeated for each encoded data and across all acquisitions until the final FOV image is built up.
In accordance with one aspect of the invention, a method of correcting gradient non-linearities in MR imaging is disclosed that includes acquiring MR data in motion, which include acquisition at different positions, in the presence of gradient non-linearities, and after transforming the MR data acquired into an image domain, applying a warping correction to the transformed MR data. The warp-corrected MR data is then corrected for the motion that occurred during MR data acquisition.
In accordance with another aspect of the invention, a method of correcting gradient non-linearities in moving table MR imaging is disclosed that includes translating a patient on a movable table within a magnet and acquiring MR data that is acquired in the presence of gradient non-linearities. The method also includes reconstructing the MR data point-by-point into an image, correcting any warping in the portion of the MR data that is acquired in the presence of the gradient non-linearities while in an image domain and shifting the MR data acquired to compensate for table motion.
In accordance with another aspect of the invention, the apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field. An RF transceiver system and an RF switch are controlled by a pulse module to transmit and receive RF signals to and from an RF coil assembly to acquire MR images. A movable table is also provided to translate an imaging object about the magnet and a computer is programmed to translate the movable table through the bore of the magnet and acquire MR data of any imaging object positioned on the table as the movable table is translated through the bore of the magnet. The computer is also programmed to further process the MR data acquired by one of a point-by-point or line-by-line basis and perform a 1-D Fourier Transform (FT) on the MR data in a direction of table motion. The computer then performs additional FTs on the MR data, depending on whether a 2D or 3D image is being acquired, and then apply a gradient non-linearity correction (GradWarp) function on the MR data. The image space is then filled with the MR data corrected for gradient non-linearities.
In accordance with yet another aspect of the invention, a computer program is disclosed for compensating gradient non-linearities in moving table MRI. The computer program is stored on a computer readable storage medium and includes a set of instructions which, when executed, cause the computer to move a patient table with respect to a gradient coil, acquire a plurality of MR data points, and perform a 1-D FFT to each MR data point. The computer program also causes the computer to place at least one MR data point into a matrix that is sized based on a desired image dimension sought and to fill a remainder of the matrix with zeros. The desired image dimensions sought could be a 2D or 3D image. Another FFT is then performed, the type of which is dependent on the desired image sought and then a GradWarp function is applied to the MR data points. Each MR data point is then corrected for patient table motion and added to build up an image.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.